Operating Systems Lecture 3: Process Concept

Process Concept

• Operating systems execute various programs as processes, which are programs in execution.
• A process must progress sequentially; no parallel execution of instructions occurs within a single process.
• Key components of a process include:
  • Program Code (Text Section)
  • Current Activity: Includes program counter and processor registers.
  • Stack: Stores temporary data such as function parameters, return addresses, and local variables.
  • Data Section: Contains global variables.
  • Heap: Memory allocated during runtime dynamically.

Process Activation

• A program remains passive when stored on disk (executable file); it becomes an active process when loaded into memory.
• Program execution can start from GUI mouse clicks or command-line entries.
• One program can have multiple processes, allowing for concurrent users accessing the same application.

Process State

• Processes shift between various states as they execute:
  • New: Process is being created.
  • Running: Instructions are actively executed.
  • Waiting: Process waits for an event to occur.
  • Ready: Process is prepared to be assigned to a processor.
  • Terminated: Process has completed execution.

Process Control Block (PCB)

• PCB contains vital information about each process, also known as the task control block.
• Key elements within PCB include:
  • Process State: Indicates current status (e.g., running, waiting).
  • Program Counter: Points to the next instruction to execute.
  • CPU Registers: Holds values of all process-centric registers.
  • CPU Scheduling Information: Contains priorities and scheduling queue pointers.
  • Memory-Management Information: Details memory allocated to the process.
  • Accounting Information: Tracks CPU time used, elapsed clock time since process start, and time limits.
  • I/O Status Information: Lists I/O devices allocated to the process and open files.

Threads in Processes

• Initially, processes have a single thread of execution.
• Multiple threads of control can be created, requiring storage for additional program counters in the PCB.

Process Scheduling

• The process scheduler is responsible for selecting the next process for CPU execution.
• Goals include maximizing CPU usage and maintaining scheduling queues:
  • Ready Queue: Includes processes in main memory that are ready and waiting for execution.
  • Wait Queues: Includes processes waiting for I/O or other events.
• Processes transition between various queues as they execute.

Queueing Diagram Representation

• Commonly visualized through queueing diagrams illustrating the flow of jobs between the ready queue, CPU, and I/O queues.
• Illustrates how processes interact with CPU and handle I/O requests.

Context Switching

• Occurs when the CPU transitions from executing one process to another.
• Involves saving the current process state into its PCB and loading the state of the next process.
• Context switches can occur due to interrupts or system calls.